1. Field of the Invention
The present invention relates to an optical-fiber coating apparatus used in a coating process for forming a primary coating and a secondary coating on a glass optical fiber, and more particularly, to an improvement of a secondary coating device that forms the secondary coating on the primary coating.
2. Description of the Related Art
Typically, a surface of a glass optical fiber made of silica glass is coated with a primary coating having a relatively low Young's modulus. Subsequently, a secondary coating having a relatively high Young's modulus compared to that of the primary coating is formed on the primary coating. As for a resin for the primary coating and the secondary coating, an ultraviolet (UV)-curing resin or a heat-curing resin is generally used.
FIG. 3 is a schematic diagram for explaining a common drawing process for forming the primary coating and the secondary coating on a surface of the glass optical fiber.
As shown in FIG. 3, an optical fiber preform 41 is fed into a heating furnace (drawing furnace) 42 at a predetermined speed from a leading end. The optical fiber preform 41 is heated and melted, and drawn into a glass optical fiber glass optical fiber 16 having a constant outer diameter in a longitudinal direction, for example, an outer diameter of 125 micrometers.
A UV-curing resin is coated on a surface of the glass optical fiber 16 with a predetermined coating thickness by a primary coating device 43. A UV light is irradiated on the coated resin in a UV irradiating chamber 44 to cure the coated resin, forming the primary coating.
After that, a UV-curing resin is coated on a surface of the primary coating with a predetermined coating thickness by a secondary coating device 45. A UV light is irradiated on the coated resin in a UV irradiating chamber 46 to cure the coated resin, forming the secondary coating.
The optical fiber on which the secondary coating is formed is introduced by a pulling capstan or a predetermined number of guide rolls provided as appropriate, and wound around a bobbin 47 that is mounted on a winding device at a predetermined winding speed.
If a heat-curing resin is used as the resin for the primary coating and the secondary coating, heating furnaces are provided instead of the UV irradiating chamber 44 and the UV irradiating chamber 46.
In these latter days, it is considered that a series of the drawing process shown in FIG. 3 should be speeded up and a length of the optical fiber to be drawn at one time should be extended, from a viewpoint of improving a productivity of the optical fiber to reduce a manufacturing cost.
However, in the secondary coating device 45 shown in FIG. 3, when a primary-coated optical fiber 17 is introduced into a resin reservoir provided in the secondary coating device 45, a foreign substance adheres to an inner wall or a surrounding portion of an inlet hole that is formed on a nipple that introduces the primary-coated optical fiber 17 to a direction of dies, and the foreign substance grows as time goes on.
When the foreign substance adheres to the inner wall of the surrounding portion of the inlet hole of the nipple, and when it grows, an inner diameter of the inlet hole diminishes. Then, the primary-coated optical fiber 17 that is passing through the inlet hole becomes damaged, and as a result, it will not be possible to continue to manufacture a high-quality optical fiber.
In fact, the foreign substance is caused from the resin composition for the primary coating that is coated on the surface of the glass optical fiber. When the resin composition for the primary coating is cured a heat is produced by the heat of polymerization. Some compositions of the resin that are not polymerized because of a relatively low molecular weight become volatile by the heat, reach the inlet hole of the nipple following the primary-coated optical fiber 17. The compositions of the resin reached the inlet hole are concentrated and adhere to the inner wall or the surrounding portion of the inlet hole.
The adherence of the foreign substance to the inner wall or the surrounding portion of the inlet hole becomes more critical as the drawing speed becomes fast and the length of the optical fiber to be drawn at one time becomes long.
To cope with the problem, a conventional technology has tried to prevent an adherence of the foreign substance by providing a tapered inlet hole of which the diameter is increased in a direction of forwarding the primary-coated optical fiber 17 at a leading edge of the inlet hole, i.e., a leading edge of the nipple, for the primary-coated optical fiber 17 in the secondary coating device 45, and blowing off the volatiles coming on a surface of the primary coating (see, for example, Japanese Patent Application Laid-Open No. H08-239246).
However, when an experiment is performed by providing a tapered inlet hole of which the diameter is increased in the direction of forwarding the primary-coated optical fiber at the leading edge of the nipple, as the secondary coating device disclosed in Japanese Patent Application Laid-Open No. H08-239246, it is found that the volatiles coming on the surface of the primary coating cannot be blown off as expected.
As a result, because an occurrence of a damage on the surface of the primary coating caused by the foreign substance cannot be prevented for sure, a speed of manufacturing the optical fiber cannot be increased. In addition, because the coating process cannot be continued for a long time, it is not possible to extend the length of the optical fiber, either.